1. Field of the Invention
The present invention relates to structures of a high-frequency semiconductor amplifier using a high-frequency transistor such as field-effect transistor (hereinafter FET) and of a radio transmission device using the high-frequency semiconductor amplifier. In particular, the present invention relates to structures of a high-frequency amplifier applied to mobile communication equipment and microwave communication equipment except for the mobile communication equipment and of a radio transmission device using the high-frequency amplifier.
2. Description of the Background Art
A radio transmission unit of a mobile terminal device for example is constructed by assembling, on a substrate of insulator, a chip having a high-frequency transistor such as FET formed on a semiconductor substrate.
FIG. 15 is a schematic block diagram illustrating a structure of a radio transmission unit 9000 applied to a conventional mobile terminal device of the type as described above.
Referring to FIG. 15, radio transmission unit 9000 includes a high-frequency amplifier 1010 capable of operating with high efficiency (hereinafter referred to as “high-efficiency amplifier”), a non-reciprocal circuit element 1030 and a transmission line 1020 connecting high-efficiency amplifier 1010 to non-reciprocal circuit element 1030.
High-efficiency amplifier 1010 is assembled in a power amplifier module 100 having an input terminal 10 and an output terminal 20. Input terminal 10 receives a transmission signal which has undergone a predetermined modulation and upconverted to a high frequency to be transmitted. An output of non-reciprocal circuit element 1030 is finally supplied to an antenna (not shown).
High-efficiency amplifier 1010 is formed on a substrate having metallic waveguide lines (transmission lines) such as microstrip lines on an insulator like ceramics or synthetic resin as described above. Specifically, high-efficiency amplifier 1010 is assembled on the substrate from an input matching circuit 104, a chip of a first-stage amplifier 105, an inter-stage matching circuit 106, a chip of a second-stage amplifier 107, and an output matching circuit 1080 arranged in this order between input terminal 10 and output terminal 20 of module 100. Those components on the substrate are connected to the metallic waveguide lines formed in advance on the substrate. Passive elements among the components on the substrate, i.e., input matching circuit 104, inter-stage matching circuit 106 and output matching circuit 1080 may be constructed in advance from a metallic layer on the substrate as the metallic waveguide lines. Fine adjustments are thereafter made to thus constructed passive elements by changing wire connection or the like in the process of assembling.
Output matching circuit 1080 includes a harmonic processing circuit 111 and a fundamental matching circuit 114. Harmonic processing circuit 111 processes harmonics by performing impedance matching for the harmonics. Fundamental matching circuit 114 performs impedance matching for the fundamental.
Non-reciprocal circuit element 1030 includes an isolator 130 for example. An output terminal 40 of non-reciprocal circuit element 1030 is connected to an antenna of a mobile communication device or the like. Non-reciprocal circuit element 1030 in such a mobile communication device enables the amplifier to operate efficiently regardless of the state of the antenna.
One example of the non-reciprocal circuit element is described below that employs an isolator.
Non-reciprocal circuit element 1030 includes an input matching circuit 120 connected to transmission line 1020 and an isolator body 130 connected between input matching circuit 120 and output terminal 40.
High-efficiency amplifier 1010 has an output impedance of 50 ohm and isolator 1030 has an input/output impedance of 50 ohm because the transmission line which has normally been used for high-frequency equipment has its characteristic impedance formed by 50 ohm termination (ohm is hereinafter represented by Ω). The second-stage amplifier 107 has an output impedance from 1 to 10 Ω. Accordingly, fundamental matching circuit 114 is constructed of a converter circuit converting the output impedance (1-10 Ω) of the second-stage amplifier 107 into 50 Ω.
A signal supplied to input terminal 10 is amplified by high-efficiency amplifier 1010. The amplified signal is passed through transmission line 1020 with the characteristic impedance of 50 Ω and isolator 1030 to be output to the antenna. Any reflected wave generated after isolator 1030 is interrupted by isolator 1030 so that the reflected wave never returns to high-efficiency amplifier 1010. Then, high-efficiency amplifier 1010 can operate in a stable manner with its high-efficiency operation maintained.
In recent years, mobile terminal equipment has been reduced remarkably in size and weight. A major factor in development of the terminal equipment is this reduction in size and weight. The size and weight of the equipment are reduced chiefly by downsizing a battery thereof. It is important, for downsizing of the battery while a certain length of time for speech communication is maintained, to enhance the operational efficiency of the amplifier with its power consumption occupying a large proportion of the entire power consumption of the mobile terminal equipment and consequently reduce the power consumption of the mobile terminal equipment itself.
However, enhancement of the amplifier efficiency is difficult in the structure of radio transmission unit 9000 explained above due to a great loss in fundamental matching circuit 114.
For example, Japanese Patent Laying-Open No. 10-327003 titled “Irreversible Circuit Element and Composite Electronic Component” addresses this problem by efficiency improvement. This document discloses a structure for allowing impedance Z to have a relation 2 Ω<Z<12.5 Ω, where Z represents each of an output impedance of a high-efficiency amplifier, an input impedance of a non-reciprocal circuit element (isolator) and a characteristic impedance of a line connecting the high-efficiency amplifier and the non-reciprocal circuit element.
FIG. 16 is a block diagram showing a structure of a radio transmission unit 9200 using a low-impedance isolator disclosed by the above-mentioned document.
Referring to FIG. 16, radio transmission unit 9200 is formed of a low-impedance high-efficiency amplifier 101, a low-impedance transmission line 102 and a low-impedance isolator 103.
Low-impedance high-efficiency amplifier 101 has an output impedance lower than the characteristic impedance 50 Ω of the normal transmission line described above and low-impedance isolator 103 has an input impedance which is also lower than 50 Ω. On the other hand, the output impedance of isolator 103 is designed to be the normal characteristic impedance 50 Ω.
In the structure shown in FIG. 16, the output impedance of high-efficiency amplifier 101 is in the range of 1 Ω to 10 Ω (corresponding to the output impedance of the second-stage amplifier 107) for example. An input matching circuit 111 of isolator 103 adjusts the input impedance of low-impedance isolator 103 to the output impedance of high-efficiency amplifier 101.
It is thus possible in the structure shown in FIG. 16 to construct high-efficiency amplifier 101 without fundamental matching circuit. Consequently, the loss generated in the output matching circuit can be avoided to reduce power consumption of the whole structure including high-efficiency amplifier 101 and isolator 103.
Input matching circuit 111 of low-impedance isolator 103 has a so-called C-L-C π type low-pass filter 113.
Low-pass filter 113 removes a harmonic component from an output of low-impedance high-efficiency amplifier 101.
The structure shown in FIG. 16 has a problem discussed below.
Low-impedance transmission line 102 is present between high-efficiency amplifier 101 serving as a power amplifier module and low-impedance isolator 103.
The input impedance of low-impedance isolator 103 in the structure as shown in FIG. 16 changes within a frequency band.
It is supposed here that the impedance is 10 Ω at the lowest frequency fl in the band and the impedance changes to 11 Ω at the highest frequency fh.
It is further supposed that transmission line 102 has an inductance represented by L. Then, the output end of high-efficiency amplifier 101 has an impedance with respect to the isolator that is (10+j2πflL) Ω (J: imaginary unit) at frequency fl while the impedance is (11+j2πfhL) Ω at frequency fh. The variation of the impedance within the band is represented by expression (1) below:√{square root over ({1+2πL(fl−fh)2})}  (1)
Accordingly, the variation of the impedance within the band increases with increase of inductor L. As a result, amplification efficiency which is one of characteristics of high-efficiency amplifier 101 deteriorates due to a relatively great impedance variation compared with the output impedance of amplifier 101.